New product and use thereof

ABSTRACT

The present invention relates to a new pre-alloyed metal based powder, intended to be used in surface coating of metal parts. The powder is deposited using e.g. laser cladding or plasma transfer arc welding (PTA), or thermal spray (e.g. HVOF). The powder is useful for reducing friction and improving wear reducing properties of the deposited coating. Such coatings may also improve machinability. As friction or wear reducing component, inclusions of manganese sulphide or tungsten sulphide in the pre-alloyed powder may be used.

FIELD OF THE INVENTION

The present invention relates to a powder mixture of three differentpre-alloyed metal based powders, intended to be used in surface coatingof metal parts. The powder mixture is deposited using e.g. lasercladding or plasma transfer arc welding (PTA), or thermal spray (e.g.HVOF). The powder mixture is useful for reducing friction and improvingwear reducing properties of the deposited coating. Such coatings mayalso improve machinability. As friction or wear reducing component,inclusions of manganese sulphide or tungsten sulphide in the pre-alloyedpowder may be used.

BACKGROUND

Thermal surfacing i.e. thermal spray coating and overlay welding powdergrades are widely used for coating of component surfaces against wearand corrosion. Fe—, Ni— and Co— based grades are known to radicallyimprove life time of wear- and/or corrosion exposed components. However,there is still a large number of applications where component life timesneed to be improved. In addition, high prices and limited availabilityof Ni and Co on the world market also calls for longer life timeimprovement. Finally, development of new coating deposition methods likelaser cladding, cold spraying and high velocity spraying open newpossibilities for alloying, more accurate control of coating process andhigher automation, thereby calling for additional types of powders.

One approach to improve friction and wear properties may be toincorporate solid lubricant to thermal surfacing grades so that thedeposited coating includes friction and wear reducing substances whilemaintaining acceptable levels of corrosion resistance and hardness.

Solid lubricants are soft solid phase materials which are capable ofreducing friction and wear between two surfaces sliding against eachother without the need for a liquid media. Materials to be considered assolid lubricants need to meet at least the following criteria: adherecontacting surfaces—stickiness: low shear strength—low intrinsicfriction; low hardness—low abrasivity and thermochemical stability forthe intended environment. Examples of solid lubricants are; talc,graphite, manganese sulphide (MnS), molybdenum disulphide (MoS₂), ortungsten disulphide (WS₂). Use of solid lubricants may provideadvantages in: stability at extremely low or high temperatures;stability in extreme environments, such as cold or hot environments, orenvironments having high radiation levels; mechanical design issues(lighter design, reduced critical velocity) or able to carry extremeloads.

For a long time, the use of solid lubricants in thermal surfacing hasbeen a difficult proposition, the reason being that numerous solidlubricants are metal sulphides and that even trace amounts of sulphur inwelds can lead to cracking and/or corrosion.

Skarvelis et al; ASME J. Tribol. 132 (2010) 031302-1-031302-8, Surf. &Coat. Techn. 203 (2009) 1384-1394, and Trib. Int. 42 (2009) 1765-1770describe the use of mixing MnS powder with a metal powder and using theresulting powder mix in e.g. PTA (plasma transferred arc welding).

An additional example of using metal powder in conjunction to MnS assolid lubricant is disclosed in Senad et al; WO2014090922.

Solid lubricants, however may have high friction coefficient compared tothat of oil or grease; finite wear life for solid lubricant films whenrenewal is not possible; no or limited cooling capacity compared to oilor grease, or tendency to clogging caused by debris and residualparticles.

SUMMARY OF THE INVENTION

It may be possible to add e.g. manganese and sulfur as individualcomponents in a metal powder, i.e. as separate powder particles. Thesecomponents will then (when the metal powder melts) form a so-calledsolid lubricant (in this case MnS). There are, however, drawbacks ofhaving e.g. Mn and S as individual components, such as severe dusting,and formation of inhomogenous inclusions of MnS in the final surfacecoating.

The inventor of the present invention has now found that it may beadvantageous to add each of the components of the solid lubricant toseparate metal powders and then mixing the metal powders eitherconcurrently with carrying out the surface coating procedure, or priorto carrying out the surface coating procedure. In short, three powdersare mixed; one metal powder containing manganese or tungsten; one metalpowder containing sulfur; and one iron based powder to enable properratios between the various components. Mn, W, and S are pre-alloyed intheir respective powder particles. These three metal powders are thenmixed together and used in a surface coating procedure, wherein themetal particles are melted, and MnS or WS inclusions are formed in themelt (also termed melt pool).

In the case of MnS, because of MnS formation in the melt pool, the slagcannot be easily removed from the top of the melt. The slag is left onthe top or sides of the surface coating, such as an overlay weldingseam. If on the sides, the next seam will cover the slag and the slagwill not have time enough to move to the seam top. Because of this, themicrostructure of the resulting hard face includes both fine-dispersedMnS but also slag-MnS.

Surprisingly, the inventor has noticed that the powder mixture accordingto the present invention can be used in applications with high tolerancewith regard to surface quality (such as surface finish, slag formation,or dimensional variability). The resulting hard face is thus suitablefor use in heavy outdoor equipment, such as rails, wheels in rail- andtram-ways, mining-, agriculture-, oil-, gas-, and construction-tools.

FIGURES

FIG. 1 Wear rate vs. sliding velocity for S-powder clad pin and carbonsteel pin for Hertzian max. contact pressure of 500 MPa.

FIG. 2 Wear rate vs. sliding velocity for S-powder clad pin and carbonsteel pin for Hertzian max. contact pressure of 1000 MPa.

FIG. 3 SEM micrograph of S-powder clad, top of the micrograph is weartest surface.

FIG. 4 SEM micrograph of S-powder clad, top of the micrograph is weartest surface.

DETAILED DESCRIPTION

All percentages herein, and in the claims are % by weight.

The invention is a powder mixture containing;

-   -   i) atomised metal powder having the following composition; C,        0.05-0.5%; Si, 2.0-4.0%; B, 0.8-1.3%; Cr, 2-10%; Fe, 3-15%; Al,        0.3-0.5%; Mn, 5-15%; the balance being Ni;    -   ii) atomised metal powder having the following composition; C,        0.05-0.2%; Si, 2.2-2.9%; B, 0.8-1.3%; Cr, 2.8-3.45%; Fe,        1.4-2.3%; Al, 0.3-0.5%; S, 3-13%; the balance being Ni;    -   iii) atomised metal powder having the following composition; C,        0.2-0.27%; Si, 3.5%; B, 1.6; Fe, 2.5; Cr, 7.5; the balance being        Ni.

Further, the invention is a powder mixture according to the above,wherein the ratio between the powders are such that the amount of MnS is4-15%.

Further, the invention is a metal powder according to the above, whereinthe particle size of the prealloyed powder is from 45 μm to 200 mm, orfrom 50-150 μm.

The invention is also a method for surface coating metal parts, by wayof laser cladding or PTA (plasma transferred arc), with a metal powderaccording to the above, thereby producing a metal coated component.

It is previously known that solid lubricants such as MnS or WS areuseful in the field of surface coating, whereby a hard phase is formedon the surface of a substrate. MnS or WS function as a so-called solidlubricant. The present inventor has shown that a mixture of metalpowders can be used in a surface coating procedure, such as plasmatransfer arc, and by choosing the right components in the individualmetal powders, the solid lubricant can form in the resulting surfacecoating or hard phase. The metal powders may be nickel, cobalt, or ironbased.

Three atomised metal powders are used in the mixture according to theinvention; In one embodiment, Powder M may have the followingcomposition; C, 0.05-0.5%; Si, 2.0-4.0%; B, 0.8-1.3%; Cr, 2-10%; Fe,3-15%; Al, 0.3-0.5%; Mn, 5-15%; the balance being Ni. The powder wasprepared by atomisation of a melt containing the elements above in saidamounts. The resulting powder contains Mn as inclusions in a matrix ofmetal alloy. This powder is herein denoted “Powder M”;

Powder S may have the following composition; C, 0.05-0.2%; Si, 2.2-2.9%;B, 0.8-1.3%; Cr, 2.8-3.45%; Fe, 1.4-2.3%; Al, 0.3-0.5%; S, 3-13%; thebalance being Ni. The powder was prepared by atomisation of a meltcontaining the elements above in said amounts. The resulting powdercontains S as inclusions in a matrix of metal alloy. This powder isherein denoted “Powder S”; and the third powder is 1540—a standardgrade. This powder is herein denoted “Powder MP”.

Powder S, Powder Mn and powder P are mixed, in order to achieve 4-15%MnS in the final melt pool which forms in the below mentioned claddingmethods. This powder mixture is herein denoted “Mixture PM”.

The Mixture PM is especially well suited for weld cladding methods, suchas laser cladding or PTA. In addition, thermal spray, e.g. flame spray,HVOF, HVAF, coldspray, plasma spray, and the like may also be suitableapplications.

The prealloyed nickel, iron, or cobalt based powder is preferablyproduced by water or gas atomization of a melt which includes Mn, W, orS and other alloying elements chosen from the group consisting of C, Si,B, Cr, Fe, Al, Ni, Co, and V.

The particle size of the pre-alloyed powder alloy is typically from 10μm to 800 μm, or from 10 μm to 200 μm, or preferably from 15-150 μm, or50-150 μm.

In one aspect, the invention provides a method for surface coating metalparts, by way of deposition techniques such as laser cladding or PTA(plasma transferred arc); thermal spray methods such as HVOF (highvelocity oxy fuel spray), HVAF (high velocity acetylene fuel spray) orplasma spray; or by slurry methods such as centrifugal casting, with theabove mentioned metal powder.

In a further aspect, the invention also provides metal parts produced bythe above mentioned suitable for coating by the powder according to theinvention for dry friction contacts in machinery, such as e.g.industrial valves, sheet metal forming (SMF) tools, transport rollers iniron works, paper knives, and glass moulds.

EXAMPLES Example 1

Preparation of Pre-alloyed Powder

A metal powder with the following composition; C, 0.05-0.5%; Si,2.0-4.0%; B, 0.8-1.3%; Cr, 2-10%; Fe, 3-15%; Al, 0.3-0.5%; Mn, 5-15%;the balance being Ni, was prepared by atomisation of a melt containingthe elements above in said amounts. The resulting powder contains Mn asinclusions in a matrix of metal alloy. This powder is herein denoted“Powder M”.

An additional metal powder with the following composition; C, 0.05-0.2%;Si, 2.2-2.9%; B, 0.8-1.3%; Cr, 2.8-3.45%; Fe, 1.4-2.3%; Al, 0.3-0.5%; S,3-13%; the balance being Ni, was prepared by atomisation of a meltcontaining the elements above in said amounts. The resulting powdercontains S as inclusions in a matrix of metal alloy. This powder isherein denoted “Powder S”.

1540—a standard grade This powder is herein denoted “Powder M P”.

Powder S, Powder Mn and powder P are mixed, 3MA powder mix, in order toachieve 4-15% MnS.

Example 2

Application of Powder by Deposition using PTA

Pre-alloyed or pre-mixed powder was applied to test samples as follows;Powder A was deposited onto S235JRG (base structural steel) substrateplates by PTA (plasma transfer arc) with parameters set to allow for adilution of 5-15%.

Example 3

Powder S was spread by hand on substrate as a powder before fusing withthe substrate. How was the powder fuwed?

Example 4

Powder according to the invention was also applied to substrate by lasercladding. The coating from Powder S appears to result in finer inclusionsizes of MnS than when applied by PTA.

Example 5

Block on ring wear testing was performed, and shows the beneficialeffects of 3MA powder mix in a metal surface coating layer or clad. Thespecimens were rectangular blocks 10×10×50 mm where the base metal wascommonly used low carbon structural steel (EN S235 JRG, ASTM A570 Gr.36)and the surface layer was at least 0.5 mm thick in the as finishedmeasure. The test surface had a ground finish with surface roughness ofRa 0.3-0.4 μm, prepared by grinding. The counter rings ø60/R100×ø20×16mm were made of UIC 900A rail steel. The test was unlubricated i.e. dry,and the test samples were carefully cleaned and then degreased byethanol prior to testing. The testing was performed as a wear mechanismmapping trial. The test normal load was 5 and 42 N what correspond 500respective 1000 MPa in max. Hertzian contact pressure. Sliding velocitywas 0.045, 0.13, 0.37, 1.1 and 2.9 m/s. The total sliding distance was800 m. Results are shown in FIG. 1 and FIG. 2 for contact pressures of500 respective 1000 MPa. FIG. 3 and FIG. 4 illustrate microstructure ofS-powder laser clad.

1. A powder mixture containing: i) atomised metal powder having thefollowing composition: C, 0.05-0.5%; Si, 2.0-4.0%; B, 0.8-1.3%; Cr,2-10%; Fe, 3-15%; Al, 0.3-0.5%; Mn, 5-15%; the balance being Ni, ii)atomised metal powder having the following composition: C, 0.05-0.2%;Si, 2.2-2.9%; B, 0.8-1.3%; Cr, 2.8-3.45%; Fe, 1.4-2.3%; Al, 0.3-0.5%; S,3-13%; the balance being Ni, and iii) atomised metal powder having thefollowing composition: C, 0.2-0.27%; Si, 3.5%; B, 1.6; Fe, 2.5; Cr, 7.5;the balance being Ni.
 2. The powder mixture according to claim 1,wherein the ratio between the powders are such that the amount of MnS is4-15%.
 3. A metal powder according to claim 1, wherein the particle sizeof the prealloyed powder is from 45 μm to 200 mm.
 4. A method forsurface coating metal parts, by way of laser cladding or PTA (plasmatransferred arc), with a metal powder according to claim 1 therebyproducing a metal coated component.